Hemodynamic effects of spontaneous breathing in the post-operative period

A Simple Method for Noninvasive Quantification of Pressure Gradient Across the Pulmonary Valve

Pressure gradient across the pulmonary valve (PVPG) is an important hemodynamic variable used in the management of patients with cardiovascular and pulmonary disease. However, a reliable noninvasive method is unavailable. We hypothesized that a progressive Muller maneuver would elicit the pulmonary valve premature opening (PVPO) in diastole and that this event would be detectable by Doppler echocardiography. The intrathoracic pressure (ITP) decrease during this maneuver equals PVPG, which may be assessed with a custom airway pressure measurement device. A total of 102 subjects were enrolled in the study. At the earliest appearance of PVPO, the ITP decrease was recorded as the PVPG. PVPG was also simultaneously measured and compared by other two methods: right heart catheterization in 43 subjects, and routine Doppler echocardiography (pulmonary regurgitation jet) in the other 59 subjects. The results measured by different approaches were compared using the Bland-Altman analysis. PVPG assessed via PVPO showed strong agreement with PVPG measured by catheterization or routine Doppler echocardiography methods, with Lin concordance correlation coefficients of 0.91 and 0.70, respectively. In conclusion, PVPO provides a new noninvasive method of quantification of PVPG.

Airway pressure release ventilation reduces the increase in bronchoalveolar lavage fluid high-mobility group box-1 levels and lung water in experimental acute respiratory distress syndrome induced by lung lavage

BACKGROUND AND OBJECTIVE

Airway pressure release ventilation (APRV) may provide better alveolar recruitment at a lower peak airway pressure than conventional mechanical ventilation (CMV) and, therefore, decrease the risk of barotrauma in patients with acute lung injury and acute respiratory distress syndrome. The present study compared the effects of APRV with low tidal volume ventilation (LTV) and CMV on the ongoing response in lung injury induced by whole lung lavage.

METHODS

Lung injury was induced by whole lung lavage. Twenty-one Japanese white rabbits were randomized to receive CMV (tidal volume 10 ml kg, positive end-expiratory pressure 3 cmH2O), LTV (tidal volume 6 ml kg, positive end-expiratory pressure 10 cmH2O), or APRV (Phigh 20 cmH2O, Plow 5 cmH2O). After 4 h of treatment, the lungs and heart were excised en bloc. The left lung was lavaged, and high-mobility group box-1 (HMGB1) levels were measured in the lavage. The right lung was analysed histologically and its wet-to-dry weight ratio was calculated.

RESULTS

PaO2 was decreased after the induction of lung injury, but the values were significantly higher in the APRV and LTV groups after treatment than in the CMV group. Serum HMGB1 levels did not change before and after lung injury; however, bronchoalveolar lavage fluid HMGB1 levels were significantly increased at the end of the experiment (266.8 +/- 47.9 in the CMV group, 137.4 +/- 23.4 in the LTV group, and 91.2 +/- 5.4 ng ml in the APRV group). The bronchoalveolar lavage fluid HMGB1 levels after experiment were significantly lower in the APRV group than in the CMV and LTV groups (P < 0.0001 and P = 0.0391, respectively). Wet-to-dry weight ratios were also lowest in the APRV group.

CONCLUSION

APRV reduces bronchoalveolar lavage fluid HMGB1 levels and lung water and it preserves oxygenation and systemic blood pressure in experimental acute respiratory distress syndrome. The results suggest that APRV could be as protective for acute respiratory distress syndrome as LTV with positive end-expiratory pressure.

Spontaneous breathing during high-frequency oscillatory ventilation improves regional lung characteristics in experimental lung injury

BACKGROUND

Maintenance of spontaneous breathing is advocated in mechanical ventilation. This study evaluates the effect of spontaneous breathing on regional lung characteristics during high-frequency oscillatory (HFO) ventilation in an animal model of mild lung injury.

METHODS

Lung injury was induced by lavage with normal saline in eight pigs (weight range 47-64 kg). HFO ventilation was applied, in runs of 30 min on paralyzed animals or on spontaneous breathing animals with a continuous fresh gas flow (CF) or a custom-made demand flow (DF) system. Electrical impedance tomography (EIT) was used to assess lung aeration and ventilation and the occurrence of hyperinflation.

RESULTS

End expiratory lung volume (EELV) decreased in all different HFO modalities. HFO, with spontaneous breathing maintained, showed preservation in lung volume in the dependent lung regions compared with paralyzed conditions. Comparing DF with paralyzed conditions, the center of ventilation was located at 50% and 51% (median, left and right lung) from anterior to posterior and at 45% and 46% respectively, P<0.05. Polynomial coefficients using a continuous flow were -0.02 (range -0.35 to 0.32) and -0.01 (-0.17 to 0.23) for CF and DF, respectively, P=0.01.

CONCLUSIONS

This animal study demonstrates that spontaneous breathing during HFO ventilation preserves lung volume, and when combined with DF, improves ventilation of the dependent lung areas. No significant hyperinflation occurred on account of spontaneous breathing. These results underline the importance of maintaining spontaneous breathing during HFO ventilation and support efforts to optimize HFO ventilators to facilitate patients' spontaneous breathing.

How to measure and interpret volumetric measures of preload

PURPOSE OF REVIEW

To update the situation over the past few years on the clinical application of volumetric measures of preload in critically ill patients.

RECENT FINDINGS

Cardiac filling pressures monitoring is unreliable for assessing cardiac preload in mechanically ventilated critically ill patients. The transpulmonary dilution indicator technique was shown to better identify preload than pulmonary arterial catheterization. Measuring static preload index as intrathoracic blood volume or global end diastolic volume provides a good preload index, either in experimental or in different clinical settings.

SUMMARY

Volumetric measures of preload are good preload indexes. These data are to be interpreted together with the clinical patient's condition, conventional hemodynamic data and the course of illness in critically ill patients. In order to evaluate whether the application of a predefined therapy algorithm based on volumetric monitoring can improve patients' outcome, more studies are needed.

Airway pressure release ventilation and biphasic positive airway pressure: a systematic review of definitional criteria

OBJECTIVE

The objective of this study was to identify the definitional criteria for the pressure-limited and time-cycled modes: airway pressure release ventilation (APRV) and biphasic positive airway pressure (BIPAP) available in the published literature.

DESIGN

Systematic review.

METHODS

Medline, PubMed, Cochrane, and CINAHL databases (1982-2006) were searched using the following terms: APRV, BIPAP, Bilevel and lung protective strategy, individually and in combination. Two independent reviewers determined the paper eligibility and abstracted data from 50 studies and 18 discussion articles.

MEASUREMENTS AND RESULTS

Of the 50 studies, 39 (78%) described APRV, and 11 (22%) described BIPAP. Various study designs, populations, or outcome measures were investigated. Compared to BIPAP, APRV was described more frequently as extreme inverse inspiratory:expiratory ratio [18/39 (46%) vs. 0/11 (0%), P = 0.004] and used rarely as a noninverse ratio [2/39 (5%) vs. 3/11 (27%), P = 0.06]. One (9%) BIPAP and eight (21%) APRV studies used mild inverse ratio (>1:1 to < or =2:1) (P = 0.7), plus there was increased use of 1:1 ratio [7 (64%) vs. 12 (31%), P = 0.08] with BIPAP. In adult studies, the mean reported set inspiratory pressure (PHigh) was 6 cm H2O greater with APRV when compared to reports of BIPAP (P = 0.3). For both modes, the mean reported positive end expiratory pressure (PLow) was 5.5 cm H2O. Thematic review identified inconsistency of mode descriptions.

CONCLUSIONS

Ambiguity exists in the criteria that distinguish APRV and BIPAP. Commercial ventilator branding may further add to confusion. Generic naming of modes and consistent definitional parameters may improve consistency of patient response for a given mode and assist with clinical implementation.

Single transpulmonary thermodilution in off-pump coronary artery bypass grafting: haemodynamic changes and effects of different anaesthetic techniques

BACKGROUND

Off-pump coronary artery bypass grafting (OPCAB) can be associated with severe cardiovascular changes, thus requiring advanced haemodynamic monitoring. Our aim was to investigate the feasibility of transpulmonary single thermodilution (STD) combined with pulse-contour analysis, a newly introduced method for cardiovascular monitoring, for assessment of changes in haemodynamics during different anaesthetic techniques in OPCAB.

METHODS

Thirty-six patients scheduled for elective OPCAB were randomized to receive anaesthesia either with midazolam, propofol or isoflurane, in addition to fentanyl and pipecuronium. After catheterization of the femoral artery, haemodynamic parameters were assessed using STD and pulse-contour analysis. The measurements were performed after induction of anaesthesia, during surgery and at 2, 4 and 6 h post-operatively.

RESULTS

At the end of surgery, the global ejection fraction decreased by 29% and 19% in the midazolam and the propofol groups, respectively, (P < 0.05) but remained unchanged in the isoflurane group. Moreover, in the isoflurane group, the left ventricular contractility index was higher and the mean arterial pressure (MAP) and the systemic vascular resistance index (SVRI) decreased in comparison with pre-operative values. Post-operatively, the cardiac index (CI) and the cardiac function index (CFI) increased in all groups (P < 0.05). The peri-operative requirement for ephedrine and nitroglycerin increased in the propofol and the midazolam groups, respectively (P < 0.05).

CONCLUSION

During OPCAB, STD and pulse-contour analysis displayed changes in preload, myocardial function and afterload that gave valuable guidance for the conduct of anaesthesia, fluid management, and the administration of vasoactive agents. As assessed using STD, isoflurane within the present dose range appears to maintain myocardial performance and vascular tone better than midazolam or propofol.